I think the ball provides lovely structure but the branches and leaves are soft which means the head of the tree does flutter in the breeze. They are a great option for small spaces like courtyards because the size of the head can be controlled.
To achieve a consistent thick ‘mop’ the head of the tree should be pruned hard each year right down to the knuckle. It looks pretty harsh but don’t despair! Prune them when bare in Winter and you will have a healthy tight head of foliage when Summer comes around.
Less common but equally attractive is to leave the head of the Mop Top unpruned. Where space is not a problem this will allow for a woollier, less rounded appearance as shown in these picture of a street side cafe in Bowral and an informal driveway avenue.